Opto-electronic integrated circuits (OEICs) often require discrimination between the two polarizations of light carried on the waveguide in the OEIC. For example, as illustrated in the plan view of FIG. 1, a polarization-diversified optical receiver 10 must separately detect the TE and TM polarization modes carried on a first waveguide 12. A laterally arranged polarization splitter 14 is formed in which the first waveguide 12 is brought to within a small distance s of a second waveguide 16 and extends in parallel with it for an interaction length L, along which the second waveguide 16 is covered with a metal layer 18. The metal layer 18 metallically loads the second waveguide 16 so as to eliminate any guiding of the TM mode on it. Thereby, the TM mode stays on the first waveguide 12. The interaction length L is chosen to be one interaction length so that the TE mode is completely transferred to the second waveguide mode but the TM mode remains on the first waveguide 12. Separate photo-diode optical detectors 18 and 20 placed at the ends of the two waveguides 16 and 12 detect the optical intensities of the TE and TM modes respectively. Erman et al. disclose such a polarization-diversified receiver in "The integration of waveguides, optical devices and detectors on InP for implementation in optical diversity coherent receivers," 15th European Conference on Optical Communications (ECOC '89), volume 2, 1989, pp. 121-128. Albrecht et al. also disclose a laterally arranged polarization splitter in "TE/TM mode splitters on InGaAsP/InP," IEEE Photonics Technology Letters, volume 2, 1990, pp. 114-115.
The prior-art polarization splitter 10 suffers several disadvantages. The length L of the interaction region is typically 1 to 5 mm and is limited by the smallest gap s obtainable between the two waveguides 12 and 16, typically 2 .mu.m. The size is further increased by the necessity that waveguide bends 22 should have radii of greater than 5 mm to minimize bend radiation losses. Further, the attempt to minimize the gap s places its fabrication at the limits of lithography so that processing variations occur, thereby degrading reproducibility.
Cheung and Zah have disclosed one solution to the problems of the polarization-diversified receiver in U.S. patent application Ser. No. 07/649,659, filed Feb. 1, 1991. They dispense with the polarization splitter 14 and instead used two photo-diode detectors, arranged serially on the same waveguide. The first detector has a compressively strained quantum-well layer in the waveguide so that it detects and absorbs only the TE-mode radiation. The second detector, which may be of conventional design, detects the remaining radiation, the TM mode. Their polarization selectivity suffers from the disadvantage that it requires complex growth, namely, growth of a waveguide layer having compressively strained quantum-well region in one linear portion and at a minimum a corresponding unstrained region in another portion.